Most structural vibrational energy is undesirable because it contributes, for example, to structural failure due to metal fatigue; to human discomfort levels; or to the generation of unwanted acoustic energy. The containment or dampening of vibrational energy particularly in metallic structures therefore is of substantial interest both in the structural design process, and also during the operational lifetime of structures to address unanticipated vibration energy conditions.
Unwanted acoustic energy is a particular problem arising in various ocean subsurface operating contexts. For example, the detection of desired signals propagating in the water is more difficult if in the same environment vibrational energy adds noise and interference.
One approach to lessening vibrational energy is to apply counter-vibrations to the vibrating member. The approach often is implemented by an actuator which applies forces on the structure in synchronization with, but counter to, the internal energy.
Several methods of applying the actuating forces are practiced in the prior art. These include grounded actuators, intrastructural actuators, and external reaction mass actuators. An example of the latter is the moving coil actuator, a description of which is contained in the Mechanical Vibration and Shock Measurements, Bruel and Kjaer, p. 239 et.seq. published by K. Larson and Son.
In conventional digital vibration cancellation using moving coil actuators, a controller performs the function of calculating an actuator force value to be applied to a vibrating structure's surface over a determined time interval. The force is generated by imparting a specific acceleration to the armature, or stepped acceleration increment, sufficient to generate counter-forces to offset or counter the structure's vibrational displacements. The calculation is based inter alia upon displacement data from sensors located on the structure. In certain applications the instantaneous position of the armature in its duty cycle may also be incorporated in this calculation. By way of example, at a point in the duty cycle when the armature is in its zero displacement position, the controller energizes the armature coil to provide a first acceleration to the armature. This acceleration creates an opposite acceleration in the armature housing, which causes a displacement of the structure to which the housing is coupled, in a direction counter to the instantaneous displacement occurring in the structure due to vibration.
The armature may be accelerated further in the next force-application time slot. If the armature is positioned a sufficient distance from its approaching positive stroke limit, the controller's command for an additional acceleration can be executed. If, however, the armature is too close to its positive stroke limit, the armature will not have sufficient travel left to deliver the additional force.
Stroke limitations of the type described constrain the utility of current reaction-mass actuators, regardless of size or shape. One consequence is that existing moving coil reaction-mass actuators with unmonitored armatures can generate essentially unpredictable and thus unreliable actuating forces in many operating conditions. Unreliability, particularly during the occurrence of transients prior to control system convergence, can effectively defeat a vibration dampening objective. Another consequence is the stroke limit constraint on frequency response and associated deliverable peak force for application of periodic forces to the attached structure. For example, the stroke limit and a desired periodic peak force to be delivered to the structure via the actuator determines the lowest frequency waveform capable of being applied. Should a greater peak force be desired from the same device, the lower frequency limit must be raised to prevent the armature from reaching its stroke limits. Conversely, lowering the expected peak force will expand the usable frequency range.